Discharge lasers, especially excimer lasers, are being increasingly used in the photo lithographic manufacture of semiconductor devices. As projection lens technology has advanced, the requirements on the control of the laser light output have increased. For example, variations in the output power, central wavelength and wavelength spectrum of a laser source must be minimized to ensure consistent process performance. To achieve this control, the output wavelength of the laser must be well characterized, and remain stable and precisely controlled over a wide range of operating conditions.
Precise measurements of laser wavelength are needed to effect precise control. A typical technique for wavelength measurement directs a portion of the laser beam through an etalon to create an optical fringe pattern, where the spacing of the fringe pattern can be related to wavelength. This fringe spacing can be detected with a sensor such as a multi-element photo diode array, and the resulting measurements can be used for a wavelength feedback control system.
An etalon is an optical device consisting of two flat surfaces, held parallel to high precision, typically to within a small fraction of the laser wavelength. These surfaces could be opposite sides of a transparent optical element, which is referred to as a “solid to etalon”. In another type of construction, the etalon could be formed by the adjacent surfaces of two transparent optical elements, separated by a spacer or spacers with parallel faces. This assembly is often referred to as an “air-spaced etalon”, although the gap between the two elements could be filled with any transparent fluid.
Special coatings can be applied to the surfaces of the etalon to enhance their reflectivity at a particular wavelength, or range of wavelengths. This creates an optical cavity, in which constructive and destructive interference of light passing through the cavity can occur. The nature of this interference will depend, among other factors, upon the wavelength, spectrum, and direction of the light, the flatness, parallelism and reflectivity of the optical surfaces, and the optical path length between the two cavity surfaces. If some number of these factors can be held constant, the nature of the interference pattern produced can be measured to provide information about the other factors.
In laser wavelength measurement systems (such systems are hereafter called wavemeters), an optical module samples a laser output beam, and directs the sampled light through a suitably positioned etalon. Since spatial variations of spectral characteristics may exist in any given laser beam, the wavemeter should equally sample all portions of the beam to determine the wave characteristic of the beam. Such a wavemeter system might also provide optical components to form an image of the resulting interference pattern, a detector to record the pattern, and electronics suitable for analyzing the pattern to measure the wavelength. Such a wavemeter was described by two of the present inventors in U.S. patent application Ser. No. 09/165,593 (now U.S. Pat. No. 5,978,394) assigned to the present assignee and incorporated herein by reference.
The resulting measurement of wavelength remains extremely sensitive to small changes in the optical path length in the etalon cavity. In the case of an air-spaced etalon, this path length is affected by the spacing of the cavity surfaces, and the index of refraction of the fluid between them, both of which may be affected by temperature changes. The cavity spacing can be controlled in a number of ways, for example, by constructing the spacers from a material known to have high dimensional stability and a low and well-characterized coefficient of thermal expansion. The index of refraction of the cavity fluid is sensitive to the temperature and pressure. Often the etalon is contained in a sealed enclosure. This enclosure would necessarily include suitable windows.
The temperature or pressure of the etalon structure and the surrounding fluid can be monitored to establish the relationship between the temperature and the wavelength measurements. A compensation factor could be applied to correct the wavelength measurements for the measured temperature. However, a temperature or pressure monitor may not detect rapidly occurring thermal variations.
What is needed is a better method for illuminating an etalon in a wavemeter so as to minimize calibration variations over a wide range of laser operating conditions.